1. Field of the Invention
The present invention relates to an improvement of the projection type liquid-crystal video display device.
2. Background Art
As is well-known, a liquid-crystal display panel may be composed of a nematic liquid-crystal cell sandwiched between a pair of parallel light polarizers. The display panel will block incident light when no voltage is applied and will transmit the light when a proper voltage is applied. The long axes of liquid-crystal molecules are reoriented in a direction perpendicular to the electrodes under an applied electric field and maximum display effect is attained when the perpendicular orientation of the long axes is perfect. In fact, however, the voltage that can be applied is limited and the long axes of liquid-crystal molecules will not be reoriented to be completely perpendicular to the electrodes but are somewhat inclined with respect to the electrodes.
This means that the quantity of light that is transmitted through the liquid crystal display panel and then travels toward the eyes of the viewer of the panel will depend on the angle of incidence of illuminating light with respect to the panel and will also depend on the angle of the line of sight of the viewer. In other words, the image contrast or its aesthetic appeal will greatly vary with the angle of re-orientation of liquid-crystal molecules and the angle at which the viewer looks at the display panel.
One method for allowing the illuminating light to be transmitted through the liquid-crystal display panel in an efficient manner would be to allow parallel rays of light to fall not perpendicularly with respect to the panel but parallel to the inclination of the reoriented long axes of liquid-crystal molecules. This method will attain maximum light transmittance through the display panel but the problem that contrast is dependent on the viewing angle still remains. Furthermore, no sharp image can be obtained on the display panel if it is viewed at a perpendicular angle.
The common projection type video display device commercially available today is a video projector which projects a CRT display onto a screen through a projection lens system which has a relatively small aperture ratio of f/1.3 or f/1.0 with an approximate focal length of 100 mm. Small aperture-ratio projection lens systems known today are typically composed of six lens units each consisting of a single glass lens element or three lens units, two of which consist of two single non-spherical plastic lenses and the last one consisting of a single glass lens element. However, both types of projection lens system have the disadvantage of expensive optical material and high fabrication costs which contribute to an increase in the overall cost of the display device.
In the conventional video projector, the CRT serves as a light emitter for projecting the image onto a screen through the projection lens system. On the other hand, the liquid-crystal display panel requires a separate source of illumination light. Furthermore, light which is parallel to the inclination of the re-orientated long axes of the liquid-crystal molecules will be transmitted through the panel most efficiently.
If, as shown in FIG. 1, the liquid-crystal display panel A is disposed such that its center is in alignment with and perpendicular to the optical axis of a projection lens B, the viewing angle and effective aperture of the lens B need to cover the dimensions of the display panel A. However, the display panel A cannot be made smaller than a diagonal dimension of about 3.0 inches. If the projection lens B has a focal distance of 100 mm, an aperture ratio of f/1.3 is necessary to attain a magnification of 20 on the screen S. This means that the effective aperture of the projection lens B is as large as the one required for the conventional video projector. However, the light source C is usually divergent and not much of the light transmitting through the display panel A is parallel to the optical axis of the projection lens B. As a result, the amount of effective light which falls upon the lens B is too small to form a sharp image on the screen S, as shown in FIG. 2. In order to increase the amount of light which falls upon the projection lens B, a tilted optical system may be used which, as shown in FIG. 3, has the display panel A inclined with respect to the projection lens B. If the panel A is tilted in such a manner that the liquid-crystal molecules will be reoriented parallel to the optical axis, the light transmitted through the panel A may efficiently fall on the projection lens B. However, the screen S is also inclined with respect to the optical axis and a trapezoidal image will be formed on the screen.
An alternative method for increasing the amount of light falling upon the projection lens B is to use a shifted optical system which, as shown in FIG. 4, has the center of the display panel A offset from the optical axis of the projection lens B. In this system, light falls obliquely on the display panel A and is transmitted efficiently therethrough to fall upon the projection lens B. Unlike the tilted system, the screen S is not inclined, as shown in FIG. 5, and the image formed is not trapezoidal but, on the other hand, the viewing angle of the projection lens B will be increased.
Suppose, for example, the case where the image on a liquid-crystal display panel having a diagonal dimension of 3 inches (46 mm.times.61 mm) is projected on the screen at a magnification of 20 through a projection lens having a focal length of 100 mm. In an unshifted state, the viewing angle needs to cover 3 inches (77 mm), or the diagonal dimension of the display panel. However, if light from the display panel makes an angle of 15.degree. with the optical axis, the amount of shift is 26.8 mm and the viewing angle has to cover 4.7 inches (120 mm). This leads to the need to use an expensive projection lens which not only has a large effective aperture for receiving the light from the display panel but also provides a large angle of view.